Polypeptide having the amino acid sequence represented by SEQ ID NO: 1 (hereinafter sometimes referred to as human metastin) is a novel peptide, which was found in human placenta as a ligand to an orphan G-protein-coupled receptor protein (hOT7T175). Reportedly, human metastin has a cancer metastasis suppressing activity, because it is encoded by KiSS-1, which is a cancer metastasis suppressor gene (Nature, 411, 613-617 (2001); WO 00/24890; JPA 2000-312590). In the spontaneous metastasis model system of mouse using B16 melanoma bearing hOT7T175 forcedly expressed, a significant decrease in the number of metastin was noted when human metastin was administered, which made it clear that human metastin showed its metastasis suppressing activity also in vivo. Based on the system in which an increase in [Ca2+] concentration in Chinese hamster ovary (hereinafter also referred to as “CHO”) cells expressing the human metastin derivative hOT7T175 is used as an indicator, human metastin shows an activity equivalent to a peptide having the sequence of 45-54 amino acids in the amino acid sequence, and the C-terminal amide structure is considered to be important for the activity. Though human metastin showed a cancer metastasis suppressing activity in animal test, further detailed studies are necessary for its effects on clinical cancer in human and its physiological significance.
Any method of efficiently quantifying human metastin has not been established. Thus, in order to elucidate the physiological activities of human metastin, it has earnestly been desired to develop the assay system of detecting/quantifying human metastin in a simple manner with high sensitivity.